Typical FM receivers, such as the receiver 10 shown in FIG. 1, include a quadrature detector 12 to demodulate frequency shift keyed (FSK) signals and other types of FM signals. As is well known in the art, a quadrature detector 12, as shown in FIG. 2, accepts an input signal on line 14 into a mixer 16. The input signal is also input to an LC circuit, comprising capacitor 18 and inductor 20, and which may also include a resistor 21. Inductor 20 is typically a tunable inductor. The LC circuit feeds an input to the mixer on line 22, which is phase shifted 90 degrees from the input signal on line 14. The mixer generates an output signal on line 24, which is then typically input to a low pass filter 26, which generates an output signal on line 28.
The LC circuit of the quadrature detector has a resonant frequency that is adjusted to match the center frequency of the Intermediate Frequency (IF) of the receiver. For purposes of the description herein, an IF of 10.7 MHz, which is a standard intermediate frequency for an FM receiver, will be used as an example. For an IF of 10.7 MHz, the values of L and C are set as shown in FIG. 2 so that the resonant frequency of the LC circuit is 10.7 MHz. As is well known in the art, the resonant frequency of an LC circuit is determined by the following equation: ##EQU1##
where L is the inductance of the circuit in henries, and C is the capacitance of the circuit in farads.
However, if the temperature of the LC circuit changes, the value of the inductance L of inductor 20 and the value of the capacitance C of capacitor 18 may vary, thereby causing the resonant frequency of the LC circuit to shift. Typically, when the temperature increases, the resonant frequency decreases, and vice versa. When the resonant frequency shifts from the center frequency, then the phase shift of the signal on line 22 is not 90 degrees, but shifts between zero and 180 degrees. If the resonant frequency shifts in the quadrature detector, then an undesirable de voltage is introduced on output line 24 and output line 28, in a manner well known to those skilled in the art. FIG. 4 shows the effect of the dc offset causing distortion on the output signal.
Therefore, a need exists in the art for a circuit that provides automatic temperature stability for FM quadrature detectors so that the resonant frequency does not shift, thereby ensuring that the quadrature detector operates in the middle of its linear region.